TRENTON, N.J. – The frozen remains of two mice injected with the organism that causes plague have not been accounted for seven weeks after being discovered missing at a University of Medicine and Dentistry of New Jersey facility in Newark, the university said Friday.

The FBI investigated and determined there was no risk to public health or any indication of the terrorist link.

It wasn't the first time plague-infected mice have disappeared from the New Jersey facility. Four years ago, in September 2005, three live mice infected with bubonic plague bacteria disappeared from various cages. Officials later said they believed the rodents had died.

UMDNJ's Public Health Research Institute issued a four-paragraph statement about the December incident late Friday saying it believes the red hazardous waste bag containing the dead mice was sterilized and incinerated along with another bag.

"Although the mice in the missing bag were used in vaccine experiments involving the bacteria Yersinia pestis, the organism that causes plague, UMDNJ has no reason to believe that this situation poses a risk to the safety or health of UMDNJ staff or the community at large," the university said in its prepared statement.

University spokesman Jerry Carey said he did not know why UMDNJ waited seven weeks to disclose the missing mice.

Bryan Travers, a spokesman for the FBI office in Newark, told The Star-Ledger of Newark that the FBI determined there was "no nexus to terrorism or risk to public health."

The U.S. Centers for Disease Control and Prevention also investigated after the Dec. 19 discovery that mice were missing. A CDC spokesman had no immediate information on the status of its investigation.

Dr. David Perlin, director of the research institute, said the experiment was part of a National Institutes of Health bioterrorism program to test a vaccine for plague. The infectious disease often caused by bites from rodent fleas is of interest to researchers because of its potential for use by bioterrorists.

Perlin said when mice die during an experiment, they are double bagged, labeled then sprayed with a disinfectant before being placed in a freezer for storage, where they are kept for the duration of the research. Afterward, the bagged remains are sterilized then shipped offsite for incineration.

"Any time you are putting something wet in the freezer, there's a chance bags can stick together, and frequently they do," he said of the disinfectant-sprayed bags.

The rodents had been infected with the plague, Perlin said, but he said they posed no threat to research staff or the public in part because they were dead. They were also housed in a secure facility that follows protocols for a biohazard site, he said.

Millions of people died from plague in the Middle Ages, when homes and work places were inhabited by flea-infested rats. Antibiotics are effective against plague, but the disease can be fatal if an infected person is not treated quickly, the CDC Web site says. There is currently no vaccine.

Perlin said the institute has begun taking inventory of all logged hazardous waste bags before sterilization following the incident.